The present invention is directed to external helical ball screw members and more particularly to an inexpensive method of making an external helical ball screw thread and associated end treatments on a workpiece.
Ball screw assemblies are in common use today on a variety of devices. Typically, a ball screw assembly consists of a ball screw member having an external helical ball nut thread, a nut having an internal helical ball screw thread fitted over the ball screw member and a plurality of bearing balls disposed partially within the external helical ball nut and the internal helical ball screw. The ball screw member may include a variety of end treatments, such as, but not limited to, an end journal, a transverse bore, and a screw thread, for positioning the ball screw member relative to other machine elements and for interconnecting the ball screw member with other machine elements.
Ball screw assemblies are often used because of their characteristics of providing a good load carrying capacity, both longitudinally and transversely of the ball screw member, low frictional resistance to displacement of the ball screw member relative to the nut, accurate translational positioning of the ball screw member relative to the nut, and accurate axial positioning of the ball screw member relative to the nut as the members are relatively displaced longitudinally. Therefore, the ball screw assemblies are often used in machinery requiring accurate translational positioning between various machine elements, particularly when extreme loads are experienced.
Numerous examples of devices using ball screw assemblies may be found in the prior art. For example, U.S. Pat. No. 2,924,265 to J. Himka, issued Feb. 9, 1960, uses a ball screw assembly and a device for adjusting a vehicle seat. U.S. Pat. No. 2,930,252 to R. E. Sears, et al., issued Mar. 29, 1960, discloses a valve control using a ball screw assembly. U.S. Pat. No. 2,935,893 issued to E. Mazur on May 10, 1960 and U.S. Pat. No. 1,967,482 issued to B. F. Schmidt on July 24, 1934 each show how a ball screw assembly may be used in a steering gear. U.S. Pat. No. 3,159,046 to J. L. Harned, et al., issued Dec. 1, 1964 shows the use of a ball screw assembly in a differential. An aircraft trim control using a ball screw assembly is taught in U.S. Pat. No. 2,772,841, issued Dec. 4, 1956 to D. H. Bonsteel. A ball screw assembly is used in various lifting jacks, as exemplified by U.S. Pat. No. 28,613, issued June 5, 1860 to C. F. Spencer. Ball screw assemblies are also typically used in worm gear drives, as disclosed in U.S. Pat. No. 3,672,239, issued to G. Titt on June 27, 1972.
Another common use for ball screw assemblies is in drive mechanisms, tool holders and drill feeds for various machine tools, as exemplified by U.S. Pat. Nos. 2,957,368 to J. Hendrickson, issued Oct. 25, 1960; 3,640,147 issued to G. Fantoni on Feb. 8, 1972; and 2,375,991 issued to H. S. Hoffar on May 15, 1945.
Numerous other uses for ball screw assemblies are well known in the art.
Two methods have been used in the past for forming external helical ball screw threads on the ball screw members of ball screw assemblies. The two methods differ substantially in cost and result in external helical ball screw threads and ball screw members of substantially differing accuracy.
The first method of the prior art is used to produce commercial or industrial quality ball screw members of moderate precision. The first method involves performing a rolling operation on the external cylindrical surface of a workpiece in order to form an external helical ball screw thread on the cylindrical surface thereof. The external helical ball screw thread produced by the rolling operation defines an irregular surface which is of little value for locating the end treatments for further operations. Therefore, all further operations on end treatments of the ball screw member formed according to the first method are typically located relative to the pitch diameter of the external helical ball screw thread. Subsequently, the workpiece is hardened. While the rolling operation is inexpensive, it does not result in a very accurate external helical ball screw thread. The external helical ball screw thread produced by the first method of the prior art is of moderate precision due to the difficulty of maintaining straightness and lead accuracy during a rolling operation. The location of end treatments relative to the external helical ball screw thread is of moderate precision as a result of difficulties arising from the angular disposition, due to lead angle, of locating pins which are used to locate the end treatments relative to the pitch diameter of the external helical ball screw thread. Furthermore, the process of positioning the end treatments using locating pins substantially increases the difficulty of forming the end treatments.
The second method of the prior art is used for making high precision ball screw members. The second method involves performing a rolling operation on the cylindrical surface of the ball screw member to form an external helical ball screw thread, forming, by suitable means, the end journal or other end treatment, heat treating the workpiece, and then regrinding the external helical ball screw thread to more accurate dimensions in relation to the end journal or other end treatment of the ball screw member. While this results in a far more accurately dimensioned and positioned ball screw assembly than the first method of the prior art, described above, it is also substantially more expensive to produce a ball screw member according to the second method than it is to produce one according to the first method.
What is needed, therefore, is a new method for producing commercial or industrial quality ball screw members which avoids the difficulties associated with the first method of the prior art with locating the end treatments of the ball screw member relative to the external helical ball screw thread. The new method preferably should be easier to carry out and to automate and yet result in a ball screw member of greater accuracy than that produced by the first method of the prior art. For many commercial purposes, a ball screw member is needed having an accuracy intermediate those provided by the first and second methods of the prior art. Accordingly, what is also needed is an inexpensive method for forming an external helical ball screw in a ball screw member, which method produces an external helical ball screw thread in a workpiece of intermediate accuracy between the above described methods at a cost substantially less than that required to produce an external helical ball screw according to the second above described method of the prior art.